The present invention is directed to a dynamic brake with backlash control, and more particularly, to a dynamic brake with backlash control for use with a peristaltic pump.
Peristaltic pumps, also referred to as roller pumps, are commonly utilized in medical applications. For instance, such pumps are often employed during cardiovascular surgery to facilitate circulation of blood between a patient and a heart-lung machine. Other common medical uses are the transfer of blood between a patient and a kidney dialyzer, and intravenous feeding of IV solutions. Generally, peristaltic pumps are simply structured, generate a constant flow, and employ disposable tubes as a member for fluid transfer.
Peristaltic pumps are relatively simple in construction and typically include a housing having rollers which progressively compress a flexible tube at spaced intervals against an arcuate surface or raceway so as to flatten or locally reduce the cross-sectional area of the tube. In this manner, fluid leading to the flexible tube is continuously forced through the flexible tube by one or another of the rollers as it proceeds along the flexible tube over the arcuate surface or raceway.
A conventional roller pump 10, as shown in FIG. 1, comprises a drive mechanism 14 furnished with a drive shaft 12, a rotating shaft 16 which rotates according to the rotation of drive shaft 12, and a hollow pump head 20 fixed to a housing 18 to which drive mechanism 14 is attached. This pump head 20 integrally incorporates a bearing block 24 through which rotating shaft 16 is inserted and rotatably supported by a pair of bearings 22 and a stator 26 arranged on the upper portion of bearing block 24. On the upper surface of stator 26 is formed a recess 28 through which the upper end of rotating shaft 16 is protruded. While this recess 28 is radially and outwardly spaced at a certain distance from the outer circumferential surface of rotating shaft 16, its inner circumferential surface 28a is coaxial with rotating shaft 16.
A rotor assembly 30 is attached to the upper portion of rotating shaft 16 in such a way as to be placed inside recess 28 of stator 26 and to stay opposite the inner circumferential surface 28a thereof. This rotor 30 is fixed to rotating shaft 16 through a bolt 32, and is so constructed as to integrally rotate along with rotating shaft 16. On the outer circumferential surface of rotor 30, at least one roller 34 is arranged so as to rotate about its own axes. A tube 36 which is filled with blood or other fluid material is placed between rotor 30 and stator 26. Tube 36 is clamped between respective rollers 34, which are attached to rotor 30, and inner circumferential surface 28a of stator 26, thereby maintaining tube 36 in a closed state at the point at which it is clamped.
Thus, in a conventional roller pump 10, rotor 30 is rotated by the rotational motion of rotating shaft 16 driven by drive mechanism 14, and the clamped portions of tube 36 move according to the revolution of rollers 34 around rotating shaft 16. Therefore, fluid inside tube 36 is transferred according to the revolution of rollers 34. The rate of rotation of the rotating shaft 16 and hence the rollers 34 is normally adjustable so that the pumping rate of the fluid within tube 36 can be adjusted. However, the pumping rate can also be adjusted by adjusting the degree to which the rollers compress the flexible tube. This can be done in peristaltic pump assemblies by providing an adjustment mechanism for adjusting the distance between the axes of the rollers and hence the distance between the roller surface and the inner circumferential surface 28a of stator 26. Another important reason for peristaltic pumps to be adjustable in this fashion is that the compressibility, size, and other qualities of the flexible tube can vary considerably.
Referring also to FIG. 2, the operation of a typical roller pump 10 is illustrated. Although roller pumps are typically capable of rotating in either direction, the solid arrow in FIG. 2 indicates that roller pump 10 is rotating in a clockwise direction to force blood through the tube or fluid conduit 36. Generally, the roller pump 10 continues to rotate until the motor drive circuitry (not shown) is disabled. When this occurs, the roller pump coasts to a gradual stop. After the roller pump has come to a complete stop, it is desirable if the rollers 34a, 34b, 34c are left free to move (i.e., rotate). This is desirable because it allows the roller pump to be hand-operated (i.e., hand-cranked), if that should become necessary.
However, when the rollers are left free to move, it is common for the roller pump to experience some recoil, that is, some amount of counter rotation (e.g., 20 degrees of counter rotation) immediately after the rollers reach zero RPM. In FIG. 2, the counter rotation is depicted by the xe2x80x9cbroken linexe2x80x9d arrow. The recoil, referred to herein as backlash, is due to the fact that the rollers are left free to move, and because there is a certain amount of counter pressure in the fluid conduit which opposes the normal rotation (e.g., clockwise rotation) of the roller pump. Backlash may cause air to be introduced into the conduit. This highly undesirable condition may lead to an air embolism or even death of the patient.
Some roller pumps employ a continuously applied brake to prevent backlash due to counter rotation. A continuously applied brake is an electrical or mechanical brake which is continuously applied to stop the motor within the pump. The brake is never removed until it is deemed necessary for the pump to begin moving the rollers again, so as to move fluid in the pump. These pumps may activate the continuously applied brake as soon as the motor drive circuitry receives a signal to stop the pump. While the continuously applied brake does, to some extent, prevent backlash, it also prevents the rollers from freely moving after the rollers have stopped rotating. In this instance, the continuously applied brake would preclude the option of hand operating the roller pump.
Accordingly, there is a need in the art for an improved braking feature for a roller pump, which substantially reduces the occurrence of backlash yet allows the roller pump to be hand operated if necessary.
It is an object of the present invention to provide an intelligent, momentary dynamic brake for use in a roller pump to prevent backlash.
It is also an object of the present invention to provide intelligent, momentary dynamic braking in a roller pump without jeopardizing the ability to hand operate the pump after the rollers have stopped rotating.
In a first embodiment of the present invention, the aforementioned and other objects are achieved by a roller pump that includes means for activating a dynamic brake when the roller pump decelerates below a predefined pump speed. The pump also includes means for deactivating the dynamic brake when pressure in the fluid conduit of the roller pump subsides.
In another embodiment of the present invention, the aforementioned and other objects are achieved by a method for preventing backlash in a roller pump. The method involves determining whether the speed of the roller pump is less than a predefined roller pump speed threshold. When it has been determined that the speed of the roller pump is less than the predefined roller pump speed threshold, a dynamic brake is activated. Then, after a predefined period of time has elapsed, the dynamic brake is deactivated.